A slight drop in prey abundance can markedly reduce the reproductive success of elephant seals. This finding comes from a study conducted at a major university.
Each winter, female elephant seals give birth to a calf and then remain on the beach for four weeks, surviving entirely on stored fat until their pups are weaned and ready to rejoin the ocean for feeding. After the winter breeding period, the seals head to sea for two months before returning to the colony to molt. They then embark on a long migration that spans thousands of miles across the North Pacific, covering seven to eight months of travel.
On these foraging journeys, the seals employ varied strategies, venture at different distances from shore, dive to different depths, and chase a range of prey, including many species of fish and squid. The research showed that diving depth most strongly predicts mass gain. Seals that dive deeper tend to eat a higher energy diet than those that dive shallowly.
The weight gained during these expeditions directly influences the seal’s capacity to reproduce. A female that does not accumulate sufficient fat will not birth a calf after returning to shore. A practical threshold emerges: animals that gain less than 260 kilograms rarely breed, while those gaining more than 260 kilograms almost always produce offspring.
Survival also depends on this weight gain, and the longer a female lives, the more offspring she can have over time. Earlier work has shown that a small group of long‑lived females contribute most of the offspring in a colony. These so‑called supermothers can live up to 23 years and may bear more than 15 calves in their lifetimes, though most females do not reach that longevity and thus have far fewer offspring. The strategies behind the success of these mothers have long remained unclear.
Researchers explain that a mere 5 percent increase in foraging success could trigger a 300 percent rise in lifetime offspring production due to the impact of greater mass on both survival and the annual birth and rearing of pups.
There is concern among scientists about plans in the fishing industry to explore the deep mesopelagic zone, where seals find some of their richest calorie sources. The study demonstrates that even small shifts in food availability can significantly affect the size of the population.